Published: May 20, 2011

r 2011 American Chemical Society 5330 dx.doi.org/10.1021/ic200560b | Inorg. Chem. 2011, 50, 5330–5332

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pubs.acs.org/IC

Open-Framework Copper Adeninate Compounds withThree-Dimensional Microchannels Tailored by AliphaticMonocarboxylic AcidsSonia P�erez-Y�a~nez,† Garikoitz Beobide,† Oscar Castillo,*,† Javier Cepeda,† Antonio Luque,*,†

Andr�es T. Aguayo,‡ and Pascual Rom�an†

†Departamento de Química Inorg�anica and ‡Departamento de Ingeniería Química, Facultad de Ciencia y Tecnología,Universidad del País Vasco UPV/EHU, Apartado 644, E-48080 Bilbao, Spain

bS Supporting Information

ABSTRACT: A series of isostructural copper(II) co-ordination polymers containing the nucleobase adenineand different monocarboxylic acids as bridging ligands,[Cu2(μ3-ade)2(μ2-OOC(CH2)nCH3)2] 3 xH2O (n from 0to 5), have been prepared. Single-crystal X-ray analysis ofacetate (n = 0) and butanoate (n = 2) compounds shows acovalent three-dimensional network in which the copper-(II) centers are bridged by μ-N3,N7,N9-adeninato and μ-O,O0-carboxylato ligands, with crystallization water moleculestrapped in the pores, which are decorated by the Watson�Crick faces of the adenine. The tunable permanent porosityof guest-free compounds was confirmed by gas adsorptionmeasurements.

Coordination polymers offer a means to a novel class ofpotentially designable materials because their metrics and

chemical functionality can be carefully adjusted for specificapplications.1 In recent times, porous metal�organic frameworks(MOFs) have attracted increasing attention because they possess arich structural chemistry and excellent gas sorption properties. Inthese types of compounds, we can functionalize the pores to directtheir specific recognition capability toward small molecules, and,therefore, the emerging porous MOFs serve as novel functionalmaterials for gas storage, separation, heterogeneous catalysis, andsensing.2 That is why the judicious choice of the organic ligands is akey step in the design of multidimensional frameworks containingtransition-metal complexes as building blocks.3 In this sense, theunsubstituted adenine nucleobase is a good candidate because itcontains at least five donor sites and its versatility as a ligand is well-known.4 Additionally, it is also known that paddle-wheel cores,with carboxylato and purine derivatives as bridging ligands, are ableto build rigid and porous MOFs.5�8

We report herein the synthesis and structural and magneticcharacterization of a new family of three-dimensional metal�organic compounds, [Cu2(μ3-ade)2(μ2-OOC(CH2)nCH3)2] 3xH2O [n from 0 (acetate) to 5 (heptanoate)], whose adsorptionmeasurements have demonstrated that the length of the aliphaticchain of the caboxylate ligands modifies the porosity of the open-framework structures.

Polycrystalline samples of compounds have been prepared bythe addition of carboxylic acid to an aqueous solution containing

the nucleobase and a copper(II) salt. Good-quality octahedralsingle crystals of acetate and butanoate compounds have beenobtained by the slow diffusion of a methanolic solution of thecopper(II) salt into an aqueous solution containing adenine andcarboxylic acid (see the Supporting Information). X-ray crystalstructure analysis9 revealed that the basic building unit of thecompounds is a centrosymmetric dimer (Figure 1a) in which twocopper(II) atoms are bridged by two adenine ligands coordi-nated by their N3 and N9 nitrogen atoms and two carboxylicligands with a μ-O,O0 coordination mode. The cluster exhibits apaddle-wheel arrangement with a dihedral angle between themean plane of the two bridging ligands of ca. 84� and a Cu 3 3 3Cudistance of around 2.85 Å.

The units are cross-linked (Figure 1b) through the apicalcoordination of the imidazole N7 atom of the adeninato ligandsto four adjacent entities with a Cu 3 3 3Cu separation across theimidazole N9/N7 bridge of 5.97 Å, so that the purine nucle-obase exhibits an unusual tridentate μ-N3,N7,N9 coordinationmode.6,7,10 The linkage is reinforced by a hydrogen-bondinginteraction between the coordinated O12 oxygen atom of thecarboxylic ligand and the N6�H6A group of the Hoogsteen faceof the nucleobase to form a R1

1(7) ring, implying the copper atom.The self-assembling process directed by the metal�adeninate

linkages generates a 4-connected uninodal net with a lvt topologyand a {42.84} Schl€afli symbol, using as a node the dinuclearbuilding unit.11 The net exhibits a three-dimensional system ofintersecting cavities (Figure 2) whose effective volume comprises37% and 25% of the unit cell volume for the acetate andbutanoate compounds, respectively.12 It is interesting to note thatthe free volume is directly related to the length of the aliphaticchain, which is pointing toward the inner portion of the channels,so that a longer chain implies less free volume. The system ofmicrochannels is filled by a variable number of crystallizationwater molecules, which are engaged themselves and anchored tothe inner walls of the pores via hydrogen bonds involving thedonor pyrimidinic N1 atom and the acceptor exocyclic N6 aminogroup of the adeninato ligands. The presence of the Watson�Crick faces of the adenine molecules in the pore walls suggeststhat these coordination polymers may provide a suitable oppor-tunity for analysis of the host�guest interactions in the solidstate.13 Extended MOFs containing biological moieties (so-called

Received: March 18, 2011

5331 dx.doi.org/10.1021/ic200560b |Inorg. Chem. 2011, 50, 5330–5332

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bio-MOFs) are extensively studied to obtain information about agreat diversity of biomolecular recognition processes,14 to ana-lyze their efficiency in selective gas capture,7,8,15 and to advancein the design of artificial systems suitable for the controllablerelease of adsorbed bioactive molecules at the cellular level.16

Thermal analysis and variable-temperature X-ray diffractionstudies (see the Supporting Information) show that the powderdiffraction patterns of solvated and desolvated samples do not differsubstantially. This fact suggests the stability of the open framework,after removal of the guest molecules, up to around 250 �C.

The permanent porosity of theMOFs was studied bymeans ofmeasurement of the nitrogen adsorption isotherms at 77 K(Figure 3). All of the samples were dried under vacuum at150 �C for 8 h to eliminate solvent guest molecules prior tomeasurement. In the case of acetate, propionate, and butanoatecompounds, the adsorption curves resemble a type I isothermwith a sharp knee at low relative pressures (p/p0 ∼ 0.01),

followed by a plateau, which is characteristic of a crystallinemicroporous solid with uniform pore-size distribution. On theother hand, the pentanoate compound exhibits an isothermcorresponding to an essentially nonadsorbing solid. The surfacearea values were obtained by fitting the adsorption data to theBrunauer�Emmett�Teller equation. Table 1 gathers the experi-mental surface area andmicropore volume (t-plot method) alongwith the corresponding theoretical values extracted from thecrystal structures.

Theoretical data of the accessible surface area were calculatedby a Monte Carlo integration technique where a probe moleculewith a diameter equal to the Lennard-Jones parameter fornitrogen (3.681 Å) is “rolled” over the framework.17 Generally,the smaller experimental values are attributed to incompletesolvent removal, to partial crystal collapse, or to the presence ofimpurities. However, discrepancies among the experimental andtheoretical values can also arise from the flexibilty of the frame-work (i.e., the solvent removal, pressure, temperature, and/oradsorbate can affect the disposition of the loose aliphaticchains).18 The experimental and calculated pore volumes andsurface areas are in good accordance with the aforementioneddepletion of the effective pore volume caused by the lengtheningof the aliphatic chain.

The magnetic behavior of the reported three-dimensionalcoordination compounds has also been examined. A priori it isdominated by the presence of two magnetic exchange pathways:the interaction involving the dimeric core (resulting fromthe Cu1�N3�C4�N9�Cu1a and the Cu1�O11�C11�O12�Cu1a bridges) and one pathway through the imidazolebridge that links the dimeric entity with the adjacent ones via theaxial N7 positions. Taking into account that the Cu 3 3 3Cuseparation across this last bridge is substantially longer thanthat involving the dimeric core and that the available data forcopper(II) complexes based on bidentate μ-N7,N9-purine

Figure 1. (a) Paddle-wheel core and (b) the cross-linkage betweenthem for an acetate compound.

Figure 2. (a) Crystal packing of the acetate compound showingthe three-dimensional network of microchannels (backbone blur).(b) Picture showing a cavity with the Watson�Crick face and thealiphatic chains pointing toward it.

Figure 3. Nitrogen adsorption isotherms at 77 K for degassedcompounds.

Table 1. Experimental and Theoretical Adsorption Data

experimental theoreticala

compound area (m2/g) V (cm3/g) area (m2/g) V (cm3/g)

acetate 505 0.173 654.8 0.286

propionate 301 0.107 393.9 0.232

butanoate 202 0.073 196.0 0.177

pentanoate 8.9 0.004 63.9 0.076aModeled from the crystal data.

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nucleobases, to date, indicate very weak antiferromagneticcouplings,6 the experimental magnetic data of these three-dimensional coordination polymers have been fitted by usingthe Bleaney�Bowers19 equation (H = �JS1S2) for a dinuclearcopper(II) complex to give antiferromagnetic interactions withJ values ranging from�242 to�212 cm�1. It is well-known thatin compounds with the [Cu2L4] core (L = purine derivatives) thenonlinear NCN bridges cause antiferromagnetic coupling withJ values from�211 to�316 cm�1, which are strongly influencedby the structural parameters of the dimeric entities.20 Otherwise,experimental and theoretical studies concerning paddle-wheel[Cu2(O2CR)4X2] species, where X is an apical oxygen or anitrogen donor ligand, have also shown that a moderate or strongantiferromagnetic coupling is expected in these kinds ofcompounds.21 However, the coexistence of two types of bridgesin the same exchange magnetic pathway may either add orcounterbalance22 their effects (orbital complementarity and coun-tercomplementarity, respectively). The orbital complementarityof the coordination modes of the adenine and carboxylato ligandswithin the dimeric unit allows the presence of relatively strongantiferromagnetic couplings in these compounds.

In summary, it has been shown that the adenine nucleobase incombination with aliphatic monocarboxylic acids and a copper-(II) salt is an excellent method for the construction of robustopen frameworks with interesting adsorptive properties, whichare tuned by the length of the aliphatic chains of the organic acids.These MOFs can be good candidates for potential industrialapplications because of their high adsorptive capacity, similar tosome commercial zeolites, but also because of the method ofsynthesis, which is easy and reproducible and involves inexpen-sive starting reagents. Moreover, the presence of the Watson�Crick face of the adenine ligands in the cavity walls suggests thatthe reported compounds would be suitable for studying supra-molecular recognition processes involving molecules of potentialbiological and medical interest.

’ASSOCIATED CONTENT

bS Supporting Information. Details of the synthesis, char-acterization, X-ray analysis, and CIF files. This material isavailable free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: oscar.castillo@ehu.es (O.C.), antonio.luque@ehu.es (A.L.).

’ACKNOWLEDGMENT

Financial support from theMinisterio de Ciencia e Innovaci�on(Project MAT2008-05690/MAT), the Gobierno Vasco (IT477-10), and the Universidad del País Vasco/Euskal HerrikoUnibertsitatea (predoctoral fellowship PIFA01/2007/021) isgratefully acknowledged. We also thank technical and humansupport provided by SGIker (UPV/EHU, MICINN, GV/EJ,and ESF).

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